Method and apparatus for manufacturing polymer fiber shells via electrospinning

ABSTRACT

An apparatus for manufacturing a polymer fiber shell from liquefied polymer is provided. The apparatus includes: (a) a precipitation electrode being for generating the polymer fiber shell thereupon; (b) a dispenser, being at a first potential relative to the precipitation electrode so as to generate an electric field between the precipitation electrode and the dispenser, the dispenser being for: (i) charging the liquefied polymer thereby providing a charged liquefied polymer; and (ii) dispensing the charged liquefied polymer in a direction of the precipitation electrode; and (c) a subsidiary electrode being at a second potential relative to the precipitation electrode, the subsidiary electrode being for modifying the electric field between the precipitation electrode and the dispenser.

RELATED PATENT APPLICATION

This application is a National Phase Application of PCT/LL01/01168International Filing Date 17 Dec. 2001, which claims priority from U.S.patent application No. 09,982,017 filed 19 Oct. 2001, which claimspriority from U.S. Provisional patent application Nos. 60,276,956 filed20 Mar. 2001 and 60,256,323 filed 19 Dec. 2000.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus formanufacturing polymer fiber shells via electrospinning.

Polymer fiber shells such as tubular shaped products, are used in themedical industry for various utilities including esophageal grafts,vascular grafts, stent coats and like.

Numerous methods for manufacturing polymer fiber shells suitable formedical applications are known in the art, including, for example,various injection molding methods, mandrel assisted extrusion orformation and various weaving techniques.

Production of polymer fiber shells suitable for use as vascular graftsis particularly difficult, since such grafts must withstand high andpulsatile blood pressures while, at the same time, be elastic andbiocompatible.

Vascular grafts known in the art typically have a microporous structurethat in general allows tissue growth and cell endothelization, thuscontributing to long term engraftment and patency of the graft.

In vascular grafts, tissue ingrowth and cell endothelization istypically enhanced with increased in grafts exhibiting increasedporosity. However, increasing the porosity of vascular grafts leads to aconsiderable reduction of the mechanical and tensile strength of thegraft, and as a consequence to a reduction in the functionality thereof.

Electrospinning has been used for generating various products formedical applications, e.g., wound dressings, prosthetic devices, andvascular grafts as well as for industrial use, e.g., electrolytic celldiaphragms, battery separators, and fuel cell components. It has alreadybeen proposed to produce by electrospinning products having theappearance of shells. For example, U.S. Pat. No. 4,323,525 discloses amethod of preparing a tubular product by electrostatically spinning afiber forming material and collecting the resulting spun fibers on arotating mandrel. U.S. Pat. No. 4,552,707 discloses a varying rotationrate mandrel which controls the “anisotropy extent” of fiber orientationof the final product. Additional examples of tubular shaped products anda like are disclosed, e.g., in U.S. Pat. Nos. 4,043,331, 4,127,706,4,143,196, 4,223,101, 4,230,650 and 4,345,414.

The process of electrospinning creates a fine stream or jet of liquidthat upon proper evaporation yields a non-woven fiber structure. Thefine stream of liquid is produced by pulling a small amount of aliquefied polymer (either polymer dissolved in solvent (polymersolution) or melted polymer) through space using electrical forces. Theproduced fibers are then collected on a suitably located precipitationdevice, such as a mandrel to form tubular structures. In the case of amelted polymer which is normally solid at room temperature, thehardening procedure may be mere cooling, however other procedures suchas chemical hardening or evaporation of solvent may also be employed.

In electrospinning, an electric field with high filed lines density(i.e., having large magnitude per unit volume) may results in a coronadischarge near the precipitation device, and consequently prevent fibersfrom being collected by the precipitation device. The filed linesdensity of an electric field is determined inter alia by the geometry ofthe precipitation device; in particular, sharp edges on theprecipitation device increase the effect of corona discharge.

In addition, due to the effect of electric dipole rotation along theelectric field maximal strength vector in the vicinity of the mandrel,products with at least a section with a small radius of curvature arecoated coaxially by the fibers. Such structural fiber formationconsiderably reduces the radial tensile strength of a spun product,which, in the case of vascular grafts, is necessary for withstandingpressures generated by blood flow.

Various electrospinning based manufacturing methods for generatingvascular grafts are known in the prior art, see, for example, U.S. Pat.Nos. 4,044,404, 4,323,525, 4,738,740, 4,743,252, and 5,575,818. However,such methods suffer from the above inherent limitations which limit theuse thereof when generating intricate profile fiber shells.

Hence, although electrospinning can be efficiently used for generatinglarge diameter shells, the nature of the electrospinning processprevents efficient generation of products having an intricate profileand/or small diameter, such as vascular grafts. In particular, sinceporosity and radial strength are conflicting, prior art electrospinningmethods cannot be effectively used for manufacturing vascular graftshaving both characteristics.

There is thus a widely recognized need for, and it would be highlyadvantageous to have, a method and apparatus for manufacturing polymerfiber shells via electrospinning devoid of the above limitations.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided anapparatus for manufacturing polymer fiber shells from liquefied polymer,the apparatus comprising: (a) a precipitation electrode being forgenerating the polymer fiber shell thereupon; (b) a dispenser, being ata first potential relative to the precipitation electrode so as togenerate an electric field between the precipitation electrode and thedispenser, the dispenser being for: (i) charging the liquefied polymerthereby providing a charged liquefied polymer; and (ii) dispensing thecharged liquefied polymer in a direction of the precipitation electrode;and (c) a subsidiary electrode being at a second potential relative tothe precipitation electrode, the subsidiary electrode being formodifying the electric field between the precipitation electrode and thedispenser.

According to another aspect of the present invention there is provided amethod for forming a liquefied polymer into a non-woven polymer fibershells, the method comprising: (a) charging the liquefied polymerthereby producing a charged liquefied polymer; (b) subjecting thecharged liquefied polymer to a first electric field; (c) dispensing thecharged liquefied polymer within the first electric field in a directionof a precipitation electrode, the precipitation electrode being designedand configured for generating the polymer fiber shell; (d) providing asecond electric field being for modifying the first electric field; and(e) using the precipitation electrode to collect the charged liquefiedpolymer thereupon, thereby forming the non-woven polymer fiber shell.

According to further features in preferred embodiments of the inventiondescribed below, the first electric field is defined between theprecipitation electrode and a dispensing electrode being at a firstpotential relative to the precipitation electrode.

According to still further features in the described preferredembodiments step (c) is effected by dispensing the charged liquefiedpolymer from the dispensing electrode.

According to still further features in the described preferredembodiments the second electric field is defined by a subsidiaryelectrode being at a second potential relative to the precipitationelectrode.

According to still further features in the described preferredembodiments the subsidiary electrode serves for reducingnon-uniformities in the first electric field

According to still further features in the described preferredembodiments the subsidiary electrode serves for controlling fiberorientation of the polymer fiber shell generated upon the precipitationelectrode.

According to still further features in the described preferredembodiments the subsidiary electrode serves to minimize a volume chargegenerated between the dispenser and the precipitation electrode.

According to still further features in the described preferredembodiments the method further comprising moving the subsidiaryelectrode along the precipitation electrode during step (e).

According to still further features in the described preferredembodiments the method further comprising moving the dispensingelectrode along the precipitation electrode during step (c).

According to still further features in the described preferredembodiments the method further comprising synchronizing the motion ofthe dispensing electrode and the subsidiary electrode along theprecipitation electrode.

According to still further features in the described preferredembodiments the dispenser comprises a mechanism for forming a jet of thecharged liquefied polymer.

According to still further features in the described preferredembodiments the apparatus further comprising a bath for holding theliquefied polymer.

According to still further features in the described preferredembodiments the mechanism for forming a jet of the charged liquefiedpolymer includes a dispensing electrode.

According to still further features in the described preferredembodiments the dispenser is operative to move along a length of theprecipitation electrode.

According to still further features in the described preferredembodiments the precipitation electrode includes at least one rotatingmandrel.

According to still further features in the described preferredembodiments the rotating mandrel is a cylindrical mandrel.

According to still further features in the described preferredembodiments the rotating mandrel is an intricate-profile mandrel.

According to still further features in the described preferredembodiments the intricate-profile mandrel includes sharp structuralelements.

According to still further features in the described preferredembodiments the cylindrical mandrel is of a diameter selected from arange of 0.1 to 20 millimeters.

According to still further features in the described preferredembodiments the precipitation electrode includes at least one structuralelement selected from the group consisting of a protrusion, an orifice,a groove, and a grind.

According to still further features in the described preferredembodiments the subsidiary electrode is of a shape selected from thegroup consisting of a plane, a cylinder, a torus and a wire.

According to still further features in the described preferredembodiments the subsidiary electrode is operative to move along a lengthof the precipitation electrode.

According to still further features in the described preferredembodiments the subsidiary electrode is tilted at angle with respect toa longitudinal axis of the precipitation electrode, the angle is rangingbetween 45 and 90 degrees.

According to still further features in the described preferredembodiments the subsidiary electrode is positioned at a distance of 5–70millimeters from the precipitation electrode.

According to still further features in the described preferredembodiments the subsidiary electrode is positioned at a distance δ fromthe precipitation electrode, δ being equal to 12βR(1−V₂/V₁), where β isa constant ranging between about 0.7 and about 0.9, R is thecurvature-radius of the polymer fiber shell formed on the precipitationelectrode, V₁ is the first potential and V₂ is the second potential.

According to yet another aspect of the present invention there isprovided an apparatus for manufacturing a polymer fiber shells fromliquefied polymer, the apparatus comprising: (a) a dispenser, for: (i)charging the liquefied polymer thereby providing a charged liquefiedpolymer; and (ii) dispensing the charged liquefied polymer; and (b) aprecipitation electrode being at a potential relative to the dispenserthereby generating an electric field between the precipitation electrodeand the dispenser, the precipitation electrode being for collecting thecharged liquefied polymer drawn by the electric field, to thereby formthe polymer fiber shell thereupon, wherein the precipitation electrodeis designed so as to reduce non-uniformities in the electric field.

According to still further features in the described preferredembodiments the precipitation electrode is formed from a combination ofelectroconductive and non-electroconductive materials.

According to still further features in the described preferredembodiments a surface of the precipitation electrode is formed by apredetermined pattern of the electroconductive and non-electroconductivematerials.

According to still further features in the described preferredembodiments the precipitation electrode is formed from at least twolayers.

According to still further features in the described preferredembodiments the at least two layers include an electroconductive layerand a partial electroconductive layer.

According to still further features in the described preferredembodiments the partial electroconductive layer is partialelectroconductive layer is formed from a combination of anelectroconductive material and at least one dielectric material.

According to still further features in the described preferredembodiments the dielectric material is selected from a group consistingof polyamide and polyacrylonitrile and polytetrafluoroethylene.

According to still further features in the described preferredembodiments the dielectric material is Titanium Nitride.

According to still further features in the described preferredembodiments the partial electroconductive layer, is selected of athickness ranging between 0.1 to 90 microns.

The present invention successfully addresses the shortcomings of thepresently known configurations by providing an electrospinning apparatusand method capable of fabricating a non-woven polymer fiber shell whichcan be used in vascular grafts.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings. With specific reference now tothe drawings in detail, it is stressed that the particulars shown are byway of example and for purposes of illustrative discussion of thepreferred embodiments of the present invention only, and are presentedin the cause of providing what is believed to be the most useful andreadily understood description of the principles and conceptual aspectsof the invention. In this regard, no attempt is made to show structuraldetails of the invention in more detail than is necessary for afundamental understanding of the invention, the description taken withthe drawings making apparent to those skilled in the art how the severalforms of the invention may be embodied in practice.

In the drawings:

FIG. 1 is a schematic illustration of a prior art electrospinningapparatus;

FIG. 2 is a schematic illustration of an electrospinning apparatus whichincludes a subsidiary electrode according to the teachings of thepresent invention;

FIG. 3 is a schematic illustration of an electrospinning apparatus whichincludes a planar subsidiary electrode according to the teachings of thepresent invention;

FIG. 4 is a schematic illustration of an electrospinning apparatus whichincludes a cylindrical subsidiary electrode according to the teachingsof the present invention;

FIG. 5 is a schematic illustration of an electrospinning apparatus whichincludes a linear subsidiary electrode according to the teachings of thepresent invention;

FIG. 6 is a schematic illustration of an electrospinning apparatus whichincludes a composite subsidiary electrode according to the teachings ofthe present invention;

FIG. 7 is an electron microscope image of material spun usingconventional electrospinning techniques;

FIG. 8 is an electron microscope image of material spun using anapparatus which incorporates a flat subsidiary electrode, positioned 20millimeters from the mandrel, according to the teachings of the presentinvention;

FIG. 9 is an electron microscope image of material spun using anapparatus which incorporates a flat subsidiary electrode, positioned 9millimeters from the mandrel, according to the teachings of the presentinvention; and

FIG. 10 is an electron microscope image of polar-oriented material spunusing an apparatus which incorporates a linear subsidiary electrodeaccording to the teachings of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is of a method and an apparatus for manufacturinga polymer fiber shell using electrospinning. Specifically, the presentinvention can be used to manufacture intricate-profile products andvascular grafts of small to large diameter via electrospinning.

For purposes of better understanding the present invention, asillustrated in FIGS. 2–10 of the drawings, reference is first made tothe construction and operation of a conventional (i.e., prior art)electrospinning apparatus as illustrated in FIG. 1.

FIG. 1 illustrates an apparatus for manufacturing a tubular structureusing a conventional electrospinning apparatus, which is referred toherein as apparatus 10.

Apparatus 10 includes a dispenser 12 which can be, for example, a bathprovided with capillary apertures 14. Dispenser 12 serves for storingthe polymer to be spun in a liquid form. Dispenser 12 is positioned at apredetermined distance from a precipitation electrode 16.

Precipitation electrode 16 serves for generating the tubular structurethereupon. Precipitation electrode 16 is typically manufactured in theform of a mandrel or any other cylindrical structure. Precipitationelectrode 16 is rotated by a mechanism such that a tubular structure isformed when coated with the polymer.

Dispenser 12 is typically grounded, while precipitation electrode 16 isconnected to a source of high voltage preferably of negative polarity,thus forming an electric field between dispenser 12 and precipitationelectrode 16. Alternatively, precipitation electrode 16 can be groundedwhile dispenser 12 is connected to a source of high voltage, preferablywith positive polarity.

To generate a tubular structure, a liquefied polymer (e.g., meltedpolymer or dissolved polymer) is extruded, for example under the actionof hydrostatic pressure, through capillary apertures 14 of dispenser 12.As soon as meniscus forms from the extruded liquefied polymer, a processof solvent evaporation or cooling starts which is accompanied by thecreation of capsules with a semi-rigid envelope or crust. An electricfield, occasionally accompanied a by unipolar corona discharge in thearea of dispenser 12, is generated by the potential difference betweendispenser 12 and precipitation electrode 16. Because the liquefiedpolymer possesses a certain degree of electrical conductivity, theabove-described capsules become charged. Electric forces of repulsionwithin the capsules lead to a drastic increase in hydrostatic pressure.The semi-rigid envelopes are stretched, and a number of pointmicro-ruptures are formed on the surface of each envelope leading tospraying of ultra-thin jets of liquefied polymer from dispenser 12.

The charges tend to distribute along the jets, thus preventing existenceof any non-zero component of electric field inside the jet. Thus, aconduction current flows along the jets, which results in theaccumulation of (different sign) free charges on the liquefied polymersurface.

Under the effect of a Coulomb force, the jets depart from the dispenser12 and travel towards the opposite polarity electrode, i.e.,precipitation electrode 16. Moving with high velocity in theinter-electrode space, the jet cools or solvent therein evaporates, thusforming fibers which are collected on the surface of precipitationelectrode 16. Since electrode 16 is rotating the charged fibers form atubular shape.

When using mandrels being at least partially with small radius ofcurvature, the orientation of the electric field maximal strength vectoris such that precipitation electrode 16 is coated coaxially by thefibers. Thus, small diameter products, have limited radial strength whenmanufactured via existing electrospinning methods, as described above.

When using mandrels with sharp edges and/or variously shaped and sizedrecesses, the electric field magnitude in the vicinity of precipitationelectrode 16 may exceed the air electric strength (about 30 kV/cm), anda corona discharge may develop in the area of precipitation electrode16. The effect of corona discharge decreases the coating efficiency ofthe process as described hereinbelow, and may even resultant in a totalinability of fibers to be collected upon precipitation electrode 16.

Corona discharge initiation is accompanied by the generation of aconsiderable amount of air ions having opposite charge sign with respectto the charged fibers. Since an electric force is directed with respectto the polarity of charges on which it acts, theses ions start to moveat the opposite direction to fibers motion i.e., from precipitationelectrode 16 towards dispenser 12. Consequently, a portion of these ionsgenerate a volume charge (ion cloud), non-uniformly distributed in theinter-electrode space, thereby causing electric field lines to partiallyclose on the volume charge rather than on precipitation electrode 16.Moreover, the existence of an opposite volume charge in theinter-electrode space, decreases the electric force on the fibers, thusresulting in a large amount of fibers accumulating in theinter-electrode space and gradually settling under gravity force. Itwill be appreciated that such an effect leads to a low-efficiencyprocess of fiber coating.

Using an infinite-length/radius cylinder as a precipitation electrode 16diminishes the effect described above. However, this effect is severeand limiting when small radii or complicated mandrels are employed forfabricating small radius or intricate-profile structures.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangement of the components setforth in the following description or illustrated in the drawings. Theinvention is capable of other embodiments or of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting.

While reducing the present invention to practice, it was uncovered thatthe use of a third electrode within an electrospinning apparatus enablesto control the electric field generated between the dispenser andprecipitation electrode. Specifically, a third electrode may eithersubstantially decreases non-uniformities in the electric field orprovides for controlled fiber orientation upon deposition.

Thus, according to the present invention there is provided an apparatusfor manufacturing a polymer fiber shell from a liquefied polymer, whichapparatus is referred to herein as apparatus 20.

As shown in FIG. 2, apparatus 20 includes a precipitation electrode 22which serves for generating the polymer fiber shell thereupon.Precipitation electrode 22 can be, for example, a mandrel of uniform orvarying radius, which may include some structural elements such as, butnot limited to, protrusions, orifices and grooves. The surface ofprecipitation electrode 22 may also contain grinds. The diameter of themandrel may vary from about 0.1 millimeter up to about 20 millimetersdepending on the diameter of the polymer fiber shell to be spunthereupon.

Apparatus 20 further includes a dispenser 24, which is at a firstpotential relative to precipitation electrode 22. Such a potential canbe generated by grounding dispenser 24, and connecting a source of highvoltage with negative polarity to precipitation electrode 22.

Alternatively, precipitation electrode 22 can be grounded whiledispenser 24 is connected to a source of high voltage with positivepolarity. In any case, an absolute value for the potential differencebetween dispenser 24 and precipitation electrode 22 may range betweenabout 10 kV and about 100 kV.

The potential difference between dispenser 24 and precipitationelectrode 22 ensures that an electric field is maintained therebetween,which electric field is important for the electrospinning process asdescribed hereinabove.

Dispenser 24 serves for charging the liquefied polymer, therebyproviding a charged liquefied polymer and dispensing the chargedliquefied polymer in a direction of precipitation electrode 22.Dispenser 24 may also include a mechanism for moving it along alongitudinal axis of precipitation electrode 22, thus enablingdispensing of the charged liquefied polymer at various points along thelongitudinal axis of precipitation electrode 22.

The charged liquefied polymer may be, for example polyurethane,polyester, polyolefin, polymethyl methacrylate, polyvinyl aromatic,polyvinyl ester, polyamide, polyimide, polyether, polycarbonate,polyacrilonitrile, polyvinyl pyrrolidone, polyethylene oxide, poly(L-lactic acid), poly (lactide-CD-glycoside), polycaprolactone,polyphosphate ester, poly (glycolic acid), poly (DL-lactic acid), andsome copolymers. Biolmolecules such as DNA, silk, chitozan and cellulosemay also be used. Improved charging of the polymer may also be required.Improved charging is effected according to the present invention bymixing the liquefied polymer with a charge control agent (e.g., adipolar additive) to form, for example, a polymer-dipolar additivecomplex which apparently better interacts with ionized air moleculesformed under the influence of the electric field. It is assumed, in anon-limiting fashion, that the extra-charge attributed to the newlyformed fibers is responsible for their more homogenous precipitation onthe precipitation electrode, wherein a fiber is better attracted to alocal maximum, which is a local position most under represented by olderprecipitated fibers, which keep their charge for 5–10 minutes. Thecharge control agent is typically added in the grams equivalent perliter range, say, in the range of from about 0.001 N to about 0.1 N,depending on the respective molecular weights of the polymer and thecharge control agent used.

U.S. Pat. Nos. 5,726,107; 5,554,722; and 5,558,809 teach the use ofcharge control agents in combination with polycondensation processes inthe production of electret fibers, which are fibers characterized in apermanent electric charge, using melt spinning and other processesdevoid of the use of an precipitation electrode. A charge control agentis added in such a way that it is incorporated into the melted orpartially melted fibers and remains incorporated therein to provide thefibers with electrostatic charge which is not dissipating for prolongedtime periods, say months.

In a preferred embodiment of the present invention, the charge controlagent transiently binds to the outer surface of the fibers and thereforethe charge dissipates shortly thereafter (within minutes). This isbecause polycondensation is not exercised at all such the chemicalintereaction between the agent and the polymer is absent, and furtherdue to the low concentration of charge control agent employed. Theresulting shell is therefore substantially charge free.

Suitable charge control agents include, but are not limited to, mono-and poly-cyclic radicals that can bind to the polymer molecule via, forexample, —C═C—, ═C—SH— or —CO—NH— groups, including biscationic amides,phenol and uryl sulfide derivatives, metal complex compounds,triphenylmethanes, dimethylmidazole and ethoxytrimethylsians.

Typically, the charged liquefied polymer is dispensed as a liquid jet,moving at high velocity under electrical forces caused by the electricfield. Thus, dispenser 24 typically includes a bath for holding theliquefied polymer and a mechanism for forming a jet, which mechanism maybe, for example, a dispensing electrode.

Apparatus 20 further includes at least one subsidiary electrode 26 whichis at a second potential relative to precipitation electrode 22.Subsidiary electrode 26 serves for controlling the direction andmagnitude of the electric field between precipitation electrode 22 anddispenser 24 and as such, subsidiary electrode 26 can be used to controlthe orientation of polymer fibers deposited on precipitation electrode22. In some embodiments, subsidiary electrode 26 serves as asupplementary screening electrode. Broadly stated, use of screeningresults in decreasing the coating precipitation factor, which isparticularly important upon mandrels having at least a section of smallradii of curvature.

The size, shape, position and number of subsidiary electrode 26 isselected so as to maximize the coating precipitation factor, whileminimizing the effect of corona discharge in the area of precipitationelectrode 22 and/or so as to provide for controlled fiber orientationupon deposition.

According to one preferred embodiment of the present invention,subsidiary electrode 26 is positioned 5–70 mm away from precipitationelectrode 22.

Preferably, such a distance is selected according to the following:δ=12βR(1−V ₂ /V ₁)  (Eq. 1)where β is a dimensionless constant named a fiber-charge accountingfactor, which ranges between about 0.7 and about 0.9, R is thecurvature-radius of precipitation electrode 22, V₁ is the potentialdifference between dispenser 24 and precipitation electrode 22 and V₂ isthe potential difference between subsidiary electrode 26 andprecipitation electrode 22.

Subsidiary electrode 26 may include a mechanism for moving it along alongitudinal axis of precipitation electrode 22. Such a mechanism may bein use when enhanced control over fiber orientation is required.

It will be appreciated that in an apparatus in which both dispenser 24and subsidiary electrode 26 are capable of such longitudinal motion,such motion may be either independent or synchronized.

Subsidiary electrode 26 may also be tilted through an angle of 45–90degrees with respect to the longitudinal axis of precipitation electrode22. Such tilting may be used to provide for controlled fiber orientationupon deposition, hence to control the radial strength of themanufactured shell; specifically, large angles result in higher radialstrength.

In addition to positioning, the shape and size of electrode 26 may alsodetermine the quality of the shell formed by apparatus 20. Thus,electrode 26 may be fabricated in a variety of shapes each serving aspecific purpose. Electrode shapes which can be used with apparatus 20of the present invention include, but are not limited to, a plane, acylinder, a torus a rod, a knife, an arc or a ring.

An apparatus 20 which includes a subsidiary electrode 26 of acylindrical (FIG. 4) or a flat shape (FIG. 3) enables manufacturingintricate-profile products being at least partially with small radius ofcurvature, which radius may range between 0.025 millimeters and 5millimeters. As can be seen in FIGS. 8–9 (further described in theExamples section), the coating of such structures is characterized byrandom-oriented (FIG. 8) or even polar-oriented (FIG. 9) fibers, asopposed to an axial coating which is typical for small curvatureproducts manufactured via existing electrospinning methods asdemonstrated in FIG. 7 (further described in the Examples section).

Preferably, when a surface of large curvature is used as subsidiaryelectrode 26, as is the case above, the distance between subsidiaryelectrode 26 and precipitation electrode 22 can be determined as δ/xwhere x is a factor ranging between 1.8 and 2, and where δ is as definedby Equation 1 above.

Thus, positioning and/or shape of electrode 26 determines fiberorientation in the polymer fiber shell formed.

The ability to control fiber orientation is important when fabricatingvascular grafts in which a high radial strength and elasticity isimportant. It will be appreciated that a polar oriented structure cangenerally be obtained also by wet spinning methods, however in wetspinning methods the fibers are thicker than those used byelectrospinning by at least an order of magnitude.

Control over fiber orientation is also advantageous when fabricatingcomposite polymer fiber shells which are manufactured by sequentialdeposition of several different fiber materials.

Reference is now made to FIG. 5, which illustrates an apparatus 20 whichutilizes a linear (e.g., a rod, a knife, an arc or a ring) subsidiaryelectrode 26.

The effect of subsidiary electrode 26 of linear shape is based on thedistortion it introduces to the electric field in an area adjacent toprecipitation electrode 22. For maximum effect the diameter ofsubsidiary electrode 26 must be considerably smaller than that ofprecipitation electrode 22, yet large enough to avoid generation of asignificant corona discharge. Fiber coating generated by apparatus 20utilizing a linear subsidiary electrode 26 is illustrated by FIG. 10which is further described in the Examples section hereinunder.

Thus, the present invention provides an electrospinning apparatus inwhich the electric field is under substantial control, thereby providingeither random or predetermined fibers orientation.

Although the use of at least one subsidiary electrode is presentlypreferred, field non-uniformities can also be at least partiallyovercome by providing a composite precipitation electrode.

As illustrated in FIG. 6, precipitation electrode 34 of apparatus 30having a dispenser 32 can be designed and configured so as to reducenon-uniformities in the electric field.

To overcome field non-uniformities, precipitation electrode 34 isfabricated from at least two layers of materials, an inner layer 36 madeof electroconductive material and an outer layer 38 made of a materialhaving high dielectric properties. Such a fabrication design results ina considerable increase of corona discharge threshold thus considerablyreducing corona discharge from precipitation electrode 34.

Materials suitable for use with outer layer 38 of precipitationelectrode 34, can be ceramic materials e.g., Titanium Nitride, AluminumOxide and the like, or polymer materials e.g., polyamide,polyacrylonitrile, polytetrafluoroethylene and the like. The thicknessof outer layer 38 depends on the dielectric properties of the materialfrom which it is made and can vary from less than one micron, in thecase of, for example, a Titanium Nitride layer, or tens of microns, inthe case of, for example, polytetrafluoroethylene, polyamide orpolyacrylonitrile layer. In addition to diminishing corona dischargethis precipitation electrode configuration enables easier separation offormed structures therefrom. Thus, according to this configuration outerlayer 38 of precipitation electrode 34 can also be configured forfacilitating the removal of the final product from the mandrel.

Additional objects, advantages, and novel features of the presentinvention will become apparent to one ordinarily skilled in the art uponexamination of the following examples, which are not intended to belimiting. Additionally, each of the various embodiments and aspects ofthe present invention as delineated hereinabove and as claimed in theclaims section below finds experimental support in the followingexamples.

EXAMPLES

Reference is now made to the following examples, which together with theabove descriptions, illustrate the invention in a non limiting fashion.

Electrospinning Material

A polycarbonate resin grade Caliper 2071 was purchased from Daw ChemicalCo. This Polymer is characterized as having good fiber forming abilitiesand is convenient for electrospinning. Chloroform was used as solvent inall of the examples described hereinbelow.

Example 1 Axial Covering Using Conventional Electrospinning Method

Reference is now made to FIG. 7, which is an example of non-randomizedcovering of thin mandrels via conventional electrospinning. A 3-mmcylindrical mandrel was covered by polycarbonate fiber using prior artelectrospinning approaches. FIG. 7 is an electron microscope image ofthe final product, in which axial fiber orientation is well evident. Dueto non-uniformities in the electric field, the fibers, while still inmotion in the inter-electrode space, are oriented in conformity with thefield configuration, and the obtained tubular structure exhibits axialorientation of fibers, and as such is characterized by axial, as opposedto radial strength.

Example 2 Random Covering Using Flat Subsidiary Electrode

An apparatus constructed and operative in accordance with the teachingsof the present invention incorporating a flat subsidiary electrodepositioned 20 millimeters from the mandrel and having the same potentialas the mandrel was used to spin a polycarbonate tubular structure of a 3mm radius. As is evident from FIG. 8, the presence of a subsidiaryelectrode randomizes fibers orientation.

Example 3 Polar-Oriented Covering Using Flat Subsidiary Electrode

An apparatus constructed and operative in accordance with the teachingsof the present invention incorporating a flat subsidiary electrodepositioned 9 millimeters from the mandrel and being at a potentialdifference of 5 kV from the mandrel was used to spin a polycarbonatetubular structure of a 3 mm radius.

As illustrated by FIG. 9, reduction of equalizing electrode-mandreldistance results in polar-oriented covering. Thus, by keeping subsidiaryelectrode and mandrel within a relatively small distance, whileproviding a non-zero potential difference therebetween, leads to slow orno fiber charge dissipation and, as a result, the inter-electrode spacebecomes populated with fiber which are held statically in a stretchedposition, oriented perpendicular to mandrel symmetry axis. Oncestretched, the fibers are gradually coiled around the rotating mandrel,generating a polar-oriented structure.

Example 4 Predefined Oriented Covering Using Linear Subsidiary Electrode

FIG. 10 illustrates result obtained from an apparatus configurationwhich may be employed in order to obtain a predefined orientedstructural fiber covering.

An apparatus which includes an elliptical subsidiary electrode and adispenser both moving along the longitudinal axis of the mandrel in areciprocating synchronous movement was used to coat a 3-mm cylindricalmandrel with polycarbonate fiber. The subsidiary electrode had a largediameter of 120 mm, a small diameter of 117.6 mm and a thickness of 1.2mm. The subsidiary electrode was positioned 15 mm from the mandrel, atan 80° tilt with respect to the mandrel symmetry axis.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention, which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable subcombination.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims. All publications, patents and patentapplications mentioned in this specification are herein incorporated intheir entirety by reference into the specification, to the same extentas if each individual publication, patent or patent application wasspecifically and individually indicated to be incorporated herein byreference. In addition, citation or identification of any reference inthis application shall not be construed as an admission that suchreference is available as prior art to the present invention.

1. An apparatus for manufacturing polymer fiber shell from liquefiedpolymer, the apparatus comprising: (a) a precipitation electrode beingfor generating the polymer fiber shell thereupon; (b) a dispenser, beingat a first potential relative to said precipitation electrode so as togenerate an electric field between said precipitation electrode and saiddispenser, said dispenser being for: (i) charging the liquefied polymerthereby providing a charged liquefied polymer; and (ii) dispensing saidcharged liquefied polymer in a direction of said precipitationelectrode; and (c) a subsidiary electrode being at a second potentialrelative to said precipitation electrode, said subsidiary electrodebeing for reducing non-uniformities in said electric field between saidprecipitation electrode and said dispenser.
 2. The apparatus of claim 1,wherein said subsidiary electrode serves for controlling fiberorientation of said polymer fiber shell generated upon saidprecipitation electrode.
 3. The apparatus according to claim 1, furthercomprising a bath for holding the liquefied polymer.
 4. The apparatusaccording to claim 1, wherein said dispenser is operative to move alonga longitudinal axis of said precipitation electrode.
 5. The apparatusaccording to claim 1, wherein said precipitation electrode includes atleast one rotating mandrel.
 6. The apparatus according to claim 5,wherein said rotating mandrel is a cylindrical mandrel.
 7. The apparatusaccording to claim 5, wherein said rotating mandrel is anintricate-profile mandrel.
 8. The apparatus according to claim 7,wherein said intricate-profile mandrel includes sharp structuralelements.
 9. The apparatus according to claim 1, wherein said subsidiaryelectrode is of a shape selected from the group consisting of a plane, acylinder, a torus and a wire.
 10. The apparatus according to claim 1,wherein said subsidiary electrode is operative to move along alongitudinal axis of said precipitation electrode.
 11. The apparatusaccording to claim 1, wherein said subsidiary electrode is tilted atangle with respect to a longitudinal axis of said precipitationelectrode, said angle is ranging between 45 and 90 degrees.
 12. A methodfor forming a liquefied polymer into a non-woven polymer fiber shell,the method comprising: (a) charging the liquefied polymer therebyproducing a charged liquefied polymer; (b) subjecting said chargedliquefied polymer to a first electric field; (c) dispensing said chargedliquefied polymer within said first electric field in a direction of aprecipitation electrode, said precipitation electrode being designed andconfigured for generating the polymer fiber shell thereupon; (d)providing a second electric field being for reducing non-uniformities insaid first electric field; and (e) using said precipitation electrode tocollect said charged liquefied polymer thereupon, thereby forming thenon-woven polymer fiber shells.
 13. The method according to claim 12,wherein said first electric field is defined between said precipitationelectrode and a dispensing electrode being at a first potential relativeto said precipitation electrode.
 14. The method according to claim 12,further comprising moving said dispensing electrode along a longitudinalaxis of said precipitation electrode during step (c).
 15. The methodaccording to claim 1, further comprising moving a subsidiary electrodealong said precipitation electrode during step (e), said subsidiaryelectrode being at a second potential relative to said precipitationelectrode so as to define said second electric field.
 16. An apparatusfor manufacturing a polymer fiber shell from liquefied polymer, theapparatus comprising: (a) a dispenser, for: (i) charging the liquefiedpolymer thereby providing a charged liquefied polymer; and (ii)dispensing said charged liquefied polymer; and (b) a precipitationelectrode being at a potential relative to said dispenser therebygenerating an electric field between said precipitation electrode andsaid dispenser, said precipitation electrode being for collecting saidcharged liquefied polymer drawn by said electric field, to thereby formthe polymer fiber shell thereupon, wherein said precipitation electrodeis formed from a combination of electroconductive andnon-electroconductive materials so as to reduce non-uniformities in saidelectric field.
 17. The apparatus of claim 1, wherein said subsidiaryelectrode serves to minimize a volume charge generated between saiddispenser and said precipitation electrode.
 18. The apparatus accordingto claim 1, wherein said dispenser and said subsidiary electrode areoperative to move synchronically along a longitudinal axis of saidprecipitation electrode.
 19. The apparatus according to claim 16,wherein a surface of said precipitation electrode is formed from apredetermined pattern of said electroconductive andnon-electroconductive materials.
 20. The apparatus according to claim16, wherein said precipitation electrode is formed from at least twolayers.
 21. The apparatus according to claim 20, wherein said at leasttwo layers include an electroconductive layer and a partialelectroconductive layer.
 22. The apparatus according to claim 21,wherein said partial electroconductive layer is formed from acombination of an electroconductive material and at least one dielectricmaterial.
 23. The apparatus according to claim 22, wherein saiddielectric material is selected from a group consisting of polyamide,polytetrafluoroethylene and polyacrylonitrile.
 24. The apparatusaccording to claim 22, wherein said dielectric material is TitaniumNitride.
 25. The apparatus according to claim 21, wherein said partiallyelectroconductive layer, is of a thickness selected from a range of 0.1to 90 microns.
 26. The apparatus according to claim 6, wherein saidprecipitation electrode is of a diameter selected from a range of 0.1 to20 millimeters.
 27. The apparatus according to claim 16, wherein saidprecipitation electrode includes at least one rotating mandrel.
 28. Theapparatus according to claim 27, wherein said rotating mandrel is acylindrical mandrel.